Biology Now, 2e

102 ■ CHAPTER 06 Cell Division

CELLS

necessary because every DNA molecule is enor-

mously long, even in the simplest eukaryotic

cells. When a chromosome is replicated, two

identical DNA molecules called sister chroma-

tids are produced. These sister chromatids are

firmly attached at a central region of the chro-

mosome called the centromere and do not sepa-

rate until metaphase (Figure 6.5).

One of the main objectives of mitosis is to

separate those sister chromatids, pulling them

apart at the centromere and delivering one of

each to the opposite ends of the parent cell.

Eukaryotic cells have evolved an elaborate chore-

ography to minimize the risk of mistakes during

phases: prophase, metaphase, anaphase, and

telophase. Each phase is defined by easily identi-

fiable events (Figure 6.4).

A parent cell sets up for an upcoming mitotic

division by replicating its DNA during the

S phase of interphase, well before mitosis gets

under way. Note that DNA in the nucleus is not

tightly packaged during the gap and synthe-

sis phases. This is because the DNA must be

accessible for replication and for conducting

the business of the cell. Then, as cell division

begins, each long, double-stranded DNA mole-

cule is attached to proteins that help pack it for

cell division into a chromosome. This packing is

The mitotic spindle aligns

the replicated chromosomes

at the center of the cell at

the metaphase plate.

The replicated chromosomes condense and

attach to the mitotic spindle, which is

made up of cytoskeletal proteins. The

nuclear envelope breaks down.

Chromosomes are^1

replicated during the

S phase, before

mitosis begins.

Sister

chromatids

Replicated, condensed

chromosomes

Metaphase

Plasma

membrane

Onion root tip

Nuclear

envelope

Fragments of

nuclear envelope

Mitotic

spindle

Interphase Mitosis

Prophase

2

C

e

ll

di

vis

ion

Inte

rp

ha

se

Unpacked

chromosomes

Figure 6.4

Cell division in a eukaryote

Mitotic cell division is composed of two main stages: mitosis (with four substages) and cytokinesis. M

Q1: Do all cells in an organism enter each stage of mitosis at the same time? (Hint: See image of onion root tip at far left in

the figure.)

Q2: What happens between the end of interphase and early prophase that changes the appearance of the chromosomes?

Q3: Explain in your own words the role of the mitotic spindle in mitosis.